1. Cross Reference to Related Applications
Copending, commonly assigned U.S. patent application Ser. No. 464,090 to Gary M. Cribbs, filed even date, and now abandoned, entitled "Method and Apparatus For Inserting An Electrically Insulated Anti-Static Circuit in a Thermoplastic Substrate" may be used in the practice of the instant invention, and the teachings thereof are hereby incorporated by reference.
2. Field of the Invention
This invention relates to a device for dissipating static charge from a substrate surface, and in particular, for preventing substantial static charge build-up on the outboard surface of a laminated aircraft window.
3. Discussion of the Technical Problem and Presently Available Techniques
Windows used in aircraft typically include several glass and/or plastic layers or plys which are fused or laminated together to form a composite transparent window. As used herein, the term composite ply, transparency, or window refers to a substrate containing more than one layer of glass or plastic or a combination of glass and plastic. In order to remove fog, mist or ice which deposits on the outside or outboard surface of the window during flight, the aircraft windows may include an internally located electrical device for heating the window. In addition to fog or ice, static electric charge also collects on the outboard surface of an aircraft window during flight. If allowed to build up, the static charge reaches an electric potential sufficient to discharge through the window to the interior heating device.
This electrical discharge often punctures small holes in the window. In extreme cases, depending on the dielectric strength of the window material, a relatively high voltage discharge can result in sufficient localized heat to severely damage the window in the vicinity of the discharge path. Further, when the discharge is through the heating device, the device can be severely damaged. It is therefore necessary to provide some facility for dissipating static charge before it reaches an electric potential sufficient to discharge through the window.
A method for preventing static charge build-up on the outboard surface of an aircraft window is to coat the outboard window surface with an electroconductive anti-static coating. Transparent electroconductive coatings of metal or metal oxide materials of suitable electroconductivity have been successfully applied to glass plys to provide an outboard anti-static coating. Typically, such coatings are applied to glass by pyrolysis of metal salts or by cathode sputtering, both of which require relatively high temperatures to yield coatings of sufficient durability. However, even though such coatings work well on glass, they are not readily usable on plastic because the high temperatures required for successful deposition of these coatings would result in deformation and consequent optical and structural distortion of presently available plastics.
As can be appreciated, plastics are used in the construction of aircraft windows because of their low density and consequent light weight. This light weight is particularly important in small aircraft where weight of the windows is significant relative to the overall weight of the aircraft. Presently available anti-static coatings which can be applied at temperatures suitable for plastic plys do not provide in service the necessary combination of optical transparency, electroconductivity and durability.
Further, anti-static coatings have additional drawbacks when used on plastic surfaces in that plastics are more likely than glass to develop scratches and other mars which interfere with visibility. Consequently, windows having an outboard plastic ply must be polished periodically to restore their smooth transparent surface. This polishing can eventually remove any coating applied to the outboard surface of a window. For this reason, an alternative to anti-static coatings is preferable for plastic aircraft windows.
U.S. Pat. No. 4,078,107 to Bitterice et al., hereby incorporated by reference, teaches a lightweight aircraft window having a heating circuit embedded the full depth of the outboard plastic ply, and an anti-static circuit embedded just beneath the outboard surface of the window. The Bitterice et al. anti-static circuit provides an electrically conductive path parallel to the outboard surface of the window for dissipation of static charge before the charge reaches a level sufficient to cause electrical discharge from the outboard surface of the transparency to the interior heating circuit. Although the Bitterice et al. circuit successfully dissipates static charge, thereby preventing damage to the heating circuit, it has limitations. For example, the Bitterice et al. circuit includes thin static charge dissipating wires parallel to the outboard surface of the window and covered with a plastic overlayer less than 5 mils (0.13 mm.) thick. Because the charge collected on the window surface must travel through the plastic overlayer to reach the dissipating wires, the charge can still create small holes or punctures therein. As can be appreciated, it would be desirable to eliminate puncturing of the plastic overlayer. Further, repeated polishing of the plastic overlayer to remove dirt and scratches may eventually wear it away, thereby exposing portions of and/or breaking the dissipating wires resulting in loss of effective charge dissipation.
Aircraft windows which are bent to conform to the aerodynamic shape of the aircraft involve a further limitation of the Bitterice et al. technique. Bending the plastic laminate having longitudinal wires close to its outboard surface may cause portions of the wire to escape through the thin plastic overlayer during the bending process, leaving these escaped portions exposed to breakage.
It is therefore desirable to have techniques for effectively dissipating static charge from the surface of nonconductive substrates which do not have the drawbacks or limitations of the presently available techniques.